As a wireless access scheme of a digital mobile phone system or a PHS system, a Time Division Multiple Access/Time Division Duplex (TDMA/TDD) scheme in which TDMA and TDD are combined has been adopted. Recently, an Orthogonal Frequency Division Multiplexing Access (OFDMA) scheme using OFDMA based on a technique of Orthogonal Frequency Division Multiplexing (OFDM) has been proposed.
The OFDM is a scheme for dividing a carrier for modulating data into a plurality of “subcarriers” (sub-divided carriers) orthogonal to each other and distributing and transmitting data signals in the subcarriers.
Next, the OFDM scheme will be schematically described.
FIG. 10 is a block diagram showing a configuration of an OFDM modulation device to be used at a transmitting side. Transmission data is input to the OFDM modulation device. The transmission data is supplied to a serial/parallel converter 201, and is converted into data configured with a plurality of low-speed transmission symbols. That is, transmission information is divided to generate a plurality of low-speed digital signals. The parallel data is supplied to an Inverse Fast Fourier Transform (IFFT) section 202.
The parallel data is allocated to OFDM subcarriers and is mapped in a frequency domain. Here, a modulation process of BPSK. QPSK, 16QAM, 64QAM, or the like is performed on the subcarriers. The mapping data is converted from frequency-domain transmission data to time-domain transmission data by performing the IFFT operation. Thereby, multi-carrier modulation signals are generated by independently modulating the plurality of subcarriers orthogonal to each other. An output of the IFFT section 202 is supplied to a guard interval adder 203.
As shown in FIG. 11, the guard interval adder 203 sets a rear part of a valid symbol of the transmission data to a guard interval and copies and adds the guard interval to a front part of a valid symbol period for every transmission symbol. A baseband signal obtained by the guard interval adder is supplied to an orthogonal modulator 204.
The orthogonal modulator 204 performs an orthogonal modulation process on the baseband OFDM signal supplied from the guard interval adder 203 using a carrier signal supplied from a local oscillator 205 of the OFDM modulation device and performs frequency conversion into an intermediate frequency (IF) or radio frequency (RF) signal. That is, the orthogonal modulator converts the baseband signal into a desired transmission frequency band and then outputs the converted signal to a transmission path.
FIG. 12 is a block diagram showing a configuration of an OFDM demodulation device to be used at a receiving side. An OFDM signal generated by the OFDM modulation device of FIG. 10 is input to the OFDM demodulation device via a predetermined transmission path.
An OFDM reception signal input to the OFDM demodulation device is supplied to an orthogonal demodulator 211. The orthogonal demodulator 211 performs an orthogonal demodulation process on the OFDM reception signal using a carrier signal supplied from a local oscillator 212 of the OFDM demodulation device, performs frequency conversion from an RF or IF signal to a baseband signal, and obtains a baseband OFDM signal. The OFDM signal is supplied to a guard interval remover 213.
The guard interval remover 213 removes a signal added by the guard interval adder 203 of the OFDM modulation device in response to a timing signal supplied from symbol timing synchronizer (not shown). The signal obtained from the guard interval remover 203 is supplied to a Fast Fourier Transform (FFT) section 214.
The FFT section 214 converts input time-domain reception data into frequency-domain reception data by means of an FFT operation. Parallel data for subcarriers is generated by demapping in the frequency domain. With this process, the subcarriers modulated by BPSK, QPSK, 16QAM, 64QAM, or the like are demodulated. The parallel data obtained from the FFT section 214 is supplied to a parallel/serial converter 215, such that the reception data is output.
The above-described OFDM is a scheme for dividing a carrier into a plurality of subcarriers. The OFDMA is a scheme for performing multiplex communication by grouping a plurality of subcarriers gathered from among OFDM subcarriers and allocating one or more groups to users. Each group is called a subchannel. That is, the users perform communication using one or more subchannels. Subchannels are adaptively increased and allocated according to an amount of communication data or a transmission environment.
Next, an example of a channel configuration in a communication system adopting the OFDMA scheme will be described.
Patent Document 1 describes a communication method by asymmetric channels whose bandwidths are different from each other in which downstream link (downlink) communication is performed by a broadband channel and upstream link (uplink) communication is performed by a narrowband channel.
FIG. 13 shows a configuration of transmission control between a terminal device and a base station in Patent Document 1. An OFDMA scheme is applied as an access scheme, and different time slots within one frame are used in time division in the upstream link and the downstream link.
A predetermined number of slots T1, T2, - - - , Tn (n is an arbitrary integer) of a first half of one frame are slots of an uplink period Tu and are slots to be used for uplink transmission from the terminal device to the base station. A predetermined number of slots, R1, R2, - - - , Rn (n is an arbitrary integer) of a second half of one frame are slots of a downlink period Td and are slots to be used for downlink transmission from the base station to the terminal device. Frames of different uplink and downlink periods (in which uplink and downlink times are different from each other and uplink and downlink slots are different from each other) are called up-down asymmetric frames.
FIG. 14 is an example of a configuration of a channel on which frame data is wirelessly transmitted. In this example, on lower and upper sides of an available frequency band B0, guard band parts B1 and B2 having narrower bandwidths than broadband channels CH1 to CH4 are present. On B1 and B2, narrowband channels CH5 and CH6 having narrower bandwidths than the broadband channels CH1 to CH4 are placed.
The narrowband channels CH5 and CH6 placed on the guard band parts are used as low-speed access dedicated communication channels in the upstream link (uplink). Only the uplink period Tu of the first half of the frame configuration shown in FIG. 13 is used for radio transmission.
Patent Document 2 describes a communication method for performing communication between a base station and a mobile station by allocating a time slot to be used to each communication party on the basis of a transmission waiting cell for each of the downstream link (downlink) and the upstream link (uplink), and describes a communication system adopting an OFDMA/TDD scheme for allocating user channels according to transmission and reception amounts and QoS of asymmetric channels. FIG. 15 is a schematic diagram showing a configuration of the communication system of Patent Document 2. Communication adopting the OFDMA scheme is performed between the base station (BTS) and the mobile station (MS).
FIG. 16 is a schematic diagram showing a frame format used in a radio communication system of Patent Document 2. As shown in FIG. 16, a unit frame (1 frame) includes an access channel Ach, an uplink control channel Cch, a downlink control channel Cch, a downlink user channel Uch, and an uplink user channel Uch.
The number of time slots included in each of the user downlink and uplink channels is not fixed and a boundary position is determined based on a user channel allocation result.
Patent Document 1: JP-A-2000-115834
Patent Document 2: JP-A-2000-236343